1. Field of the Invention
The present invention relates to a torque detecting apparatus which is capable of measuring, in a contactless manner and with high accuracy, torque applied to rotating shafts employed in various rotary machines such as, for example, steering shafts of motor vehicles, robot arms and the like.
2. Description of the Prior Art
As a means for measuring torque applied to a rotating shaft, there has been proposed a torque sensor of such a structure as shown in FIG. 24 of the accompanying drawings. In this conjunction, reference may be made to JP-A-63-36124 (Japanese Patent Application Laid-Open No. 36124/1988). As can be seen in FIG. 24, this prior art torque sensor is composed of three ring-shaped cores (magnetic iron cores) 91, 92 and 93 disposed coaxially with a twistable shaft S subjected to an amount of torsion which is variable as a function of magnitude of torque applied to the shaft. The two inner ring-shaped cores 92 and 93 are connected to the twistable shaft S at two different positions by means of respective brackets 921 and 931, wherein both the ring-shaped cores 92 and 93 have the opposite surfaces formed with indentations 92A, 92B and 93A, 93B, respectively, while the outer ring core 91 is provided with stationarily supported sensor coils 91A and 91B. In operation, the twistable shaft S undergoes torsion upon application of torque, whereby the opposing surface areas of the indentations 92A, 92B and 93A, 93B formed in the ring-shaped cores 92 and 93 are changed, causing a corresponding change in the number of lines of magnetic force 90. Consequently, inductance at the sensor coils 91A and 91B is caused to very correspondingly. Since the change in inductance corresponds to the magnitude of applied torque, it is possible to detect the latter in terms of the change in inductance.
FIG. 25 shows a torque detecting apparatus developed by the same applicant as the present application and disclosed in JP-A-62-263490. The torque detecting apparatus comprises a driving shaft 1 and a driven shaft 2 coupled together by an elastic member 10 disposed therebetween, a resonance circuit including sensor coils 99A and 99B, and a detection circuit adapted to cooperate with the resonance circuit. More specifically, the sensor coils 99A and 99B are mounted on the driving shaft 1 and the driven shaft 2, respectively, in opposition to each other. A solenoid coil 95 is wound around the driving shaft 1 and connected in series to the sensor coils 99A and 99B through a capacitor 98 to thereby constitute a resonance circuit. Additionally, an input coil 96 and an output coil 97 which constitute the detection circuit mentioned above are disposed along the outer periphery of the solenoid coil 95 with a distance therefrom. Each of the sensor coils 99A and 99B is constituted by a core of a magnetic material and an electrically conductive wire wound around the core. This torque detecting apparatus operates in the manner mentioned below. When an angular displacement takes place between the driving shaft 1 and the driven shaft 2 upon application of torque, the opposing surface areas of the sensor coils 99A and 99B are changed, resulting in a corresponding change in the number of lines of magnetic force, whereby mutual inductance of the sensor coils is caused to change, involving a change in the synthesized inductance. Thus, the magnitude of torque can be detected in terms of the corresponding change in inductance.
However, the first torque sensor in which three ring-shaped cores are employed suffers a problem in that there is temperature dependence in the inductance because a core of magnetic material is employed. In this conjunction, it is noted that although the temperature dependency at the zero point can be compensated for by virtue of a differential detecting method as adopted, the temperature dependency of the sensitivity of the torque sensor is too significant to be cancelled out.
On the other hand, in the case of the last torque detecting apparatus in which a pair of sensor coils are employed, two sets of circuits each having the configuration shown in FIG. 25 are connected such that the current flows through the sensor coils in the same direction in one detection circuit set and in opposite directions in the other set (i.e. in same phase and reverse phase, respectively), wherein the difference or ratio between the outputs from both sets is utilized as the output signal for the purpose of temperature compensation. However, since a core of magnetic material is used as the sensor coil, difference is found in the temperature dependency of respective coupling factors which determine the mutual inductance. Thus, it is impossible to adequately effectuate temperature compensation.
More specifically, the mutual inductance is determined by the coupling factor of the two sensor coils. However, when a core of magnetic material (such as ferrite or the like) is employed, the coupling factor also exhibits a temperature dependency according to the temperature dependency of the magnetic core permeability. Also, the temperature dependency varies depending on the direction of the coupling (same phase and reverse phase) as well as the degree of coupling (magnitude of the opposite surface areas). Accordingly, the difference (or ratio) between ouputs of the two sets of detecting circuits can not lead to satisfactory temperature compensation.
As will be understood from the above, both the known torque detecting apparatuses suffer from the susceptibility of temperature characteristics to the influence of temperature dependence.